FIG. 1 shows a conventional pneumatic valve actuator which includes a toothed shaft 10, an actuating shaft 20 extending through the toothed shaft 10, two piston members 30 each having a rack member 301 engaged with the toothed shaft 10, and a plurality of springs 302 biasedly disposed between an inner side of a housing 40 and the piston members 30. In operation, the pneumatic valve actuator operates on the basis of cycles of air movement. At the beginning of a cycle air under pressure enters the interior of the housing 100 via two holes 41 to push the piston members 30 from a starting position away from each other to a fully separated position (as illustrated in FIG. 1) such that the toothed shaft 10 is rotated in a counter-clockwise direction by the movement of the two rack members 301 and the springs 302 are thereby compressed. By virtue of the rotation of the toothed shaft the actuating shaft 20 is also rotated. The rotation of the actuating shaft 20 is utilized for some other function (not shown). When the piston members 30 reach the fully separated position air entry into the housing is stopped, and the two holes 41 are opened to vent the housing at which time, the springs 302 push the piston members 30 back to the original starting position and thereby the toothed shaft 10, and correspondingly, shaft 20 are rotated in the clock-wise direction. When the piston members reach the starting position, one cycle will have been completed. During operation, the force of pressurized air in the housing 100 causes leakage at the positions where the toothed shaft 10 and/or the actuating shaft 20 extend through the housing 40 (not shown in FIG. 1). Depending upon the construction characteristics and materials used in the valve, as well as the amount of pressure, even after using such actuators for a short period of time leakage can occur. Furthermore, the interior surfaces of the housing 40 and contact and sliding surfaces of the rack members 301 must be manufactured precisely to ensure that the rack members 301 slides smoothly along the inner surfaces of the housing 40 all of which increases the cost of manufacturing.
Another commonly used pneumatic valve actuator is illustrated in FIGS. 2 and 3. The actuator 6000 is disposed between a return spring 7400 and a valve 7200 with a shaft 6200 extending through the return spring, the actuator and the valve so that when pressurized air is injected into the actuator, the shaft is rotated to operate the valve.
The actuator includes a casing, including an upper casing 6010, a lower casing 6020 and a vane member 6400 which is received between the upper and lower casing. The upper and lower casing are connected by bolts 6030 along flanges extending from each of the upper and lower casing wherein the lower casing has two passages 6800 defined therein so that pressurized air can be injected from the air pump and into the passages. The shaft rotatably extends through the upper casing and the lower casing and securely extends through the vane member. A seal member 6600 is disposed to the vane member so that the piston member is reciprocally moved within the casing by pressurized air entering the casing through the passages. The shaft is co-rotated with the vane member so as to control the actuator between an open and closed position. A return spring means 7400 including a spring coil 7600 is disposed above the actuator casing in accordance with a requirement to automatically return the shaft to its starting position once the pressurized air is stopped, thereby returning the vane to its original position.
The seal member tends to become quickly worn out because the seal member slides along a inner surface of the casing whenever the piston moves. Furthermore, the inner surface of each of the upper and lower casing must be machined smooth to prolong the life of the seal. The return means including the coil spring and the machining of the inner surface of the casing results in the whole assembly being quite expensive.